Bombardment-free microwave waveguide window



Nov. 29, 1966 Filed March 18, 1963 B. VURAL BOMBARDMENT-FREE MICROWAVEWAVEGUIDE WINDOW 2 Sheets-Sheet 1 /zm/e AR T ATTORNEY N v- 29. 1 B.VURAL 3,289,122

BOMBARDMENT-FREE MICROWAVE WAVEGUIDE WINDOW Filed March 18, 1963 2Sheets-Sheet 2 INVENTOR. BA YRA M VURA L ATTORNEY United States PatentOfifice 3,289,122 Patented Nov. 29, 1966 sion Filed Mar. 18, 1963, Ser.No. 266,115 3 Claims. (Cl. 33398) The invention disclosed herein wasmade under or in the course of Contract AT(043)363 with the UnitedStates Atomic Energy Commission.

The present invention is related generally to waveguide windows and, inparticular, to a bombardment-free, microwave waveguide window whereinbombardment of the dielectric window disc by charged particles isreduced to an optimum by the configuration of the electromagnetic fieldestablished in the window region.

Vacuum windows present one of the major problems at present in thedesign of practical high-power microwave tubes and waveguide powertransmission systems. In the three-centimeter wavelength region, recentdevelopments in high-power tubes have created a need for output windowswhich have frequency bandwidths of about 15 percent, and which arecapable of transmitting peak power in excess of megawatts and averagepower in the neighborhood of 10 kilowatts. Such windows are used in theoutput waveguide of high-power tubes, such as klystrons, to permit thetransmission of RF power from the evacuated output cavities of the tubesto a relatively pressurized external circuit, e.g., the accelerator pipeof a travelingwave linear accelerator.

Most waveguide configurations which attempt to transmit high-energy RFpower from one pressure region to another pressure region therein,through a pressure isolaing window, have realized dielectric discfailures for various reasons. As one example, particularly manifested inlinear accelerator applications, waveguide window discs tend to fail dueto minute punctures or pits which seem to start on the accelerator sideof the disc and work toward the tube side, especially in cases where thewindows are required to transmit very high power signals. Such pittingand subsequent failure of the window discs can be attributed tobombardment of the dielectric disc surface by high energy electrons andions with consequent charging of the disc surfaces. A related problem isthe apparent occurrence of different surface conductivities which may beattributable to the different secondary and field emissioncharacteristics which, in turn, are due to the differences created inthe surface conditions of the window disc caused by the presence of pumpoil vapor on one side, but not on the other side, of the window.

It is possible to prevent the occurrence of such different dielectricdisc surface conductivities of above mention which occur when the discis bombarded by ions or electrons, e.g., by increasing the surfaceconductivity. However, rather than attempt to solve the problems causedby bombardment of the disc, it is preferable to prevent such bombardmentand, thus, the associated undesirable effects, and thereby circumvententirely the problem of window disc failure due to particle bombardmentthereof.

Therefore, it is an object of the present invention to provide awaveguide window wherein the bombardment of the dielectric window discby electrons and ions is substantially reduced.

It is another object of the present invention to provide a waveguidewindow wherein bombardment of window disc by electrons and ions issubstantially reduced by novel application of gradient field forces.

It is yet another object of the present invention to provide aparticularly shaped waveguide window for generating gradient fieldforces wherein particles approaching the window will experience a netacceleration in a direction away from the window disc surfaces.

It is a further object of the present invention to provide a waveguidewindow having a relatively abrupt transition region near the windowdisc, wherein the generated electric field lines bow outwardly to createan average field force which acts in a direction away from the disc.

It is still a further object of the present invention to provide awaveguide window wherein charging of the dielectric disc surfaces iscircumvented by substantially reducing the bombardment of the disc byhigh-energy electrons and ions.

Other objects and advantages will be apparent in the followingdescription and claims considered together with the accompanyingdrawings, in which:

FIGURE 1 is a cross-sectional view of the RF field configuration in thetransition region of a prior-art window having abrupt transitions fromcircular to rectangular waveguide,

FIGURE 2 is a cross-sectional view of an embodiment of the presentinvention showing the associated electric field configuration therein,

FIGURE 3 is a cross-sectional view of an alternative embodiment of thepresent invention and the associated electric field configurationtherein,

FIGURE 4 is a cross-sectional view of yet another embodiment of thepresent invention indicating a portion of the associated electric fieldconfiguration,

FIGURE 5 is a cross-sectional view of still another possible embodimentof the present invention and associated electric field configuration.

FIGURE 1 shows the usual RF field configuration (depicted by arrows 10)in the abrupt transition region 12 of a prior-art waveguide window 14-utilizing as a window disc support a length of circular waveguide 16which is secured at either end thereof to a length of rectangularwaveguide 18 of a waveguide system. As is well known in the art, thereexists a mismatch in the circular guide 16, partailly due to thepresence of a dielectric window disc 20 in the configuration ofFIGURE 1. Consequently, there will he places in the guide where theelectric field strength may be extremely high. As seen in FIGURE 1, theelectric field strength converging at the corners of the transition fromcircular to rectangular waveguide can be very high depending on thecurvature of such corners. If the curvature is Zero, the electric fieldintensity would theoretically be infinite at the corners. Obviously, inpractice the curvature has a finite value and the field strength is thusalso finite.

Thus, due to the spatially non-uniform electromagnetic field caused bythe inherent non-uniformity of the TE mode and the non-uniformitycreated by the sharp transition from the circular to rectangularwaveguide, there exists an E component in the window region, which hasits maximum value at the corners of the transition area as shown inFIGURE 1. In estimating the magnitude of the forces acting on theparticles within the electromagnetic field in the window region, thetime-average forces, F, due to the field gradients are of particularinterest.

In a spatially non-uniform RF field there are timeaverage forces ofmagnitude proportional to the gradient of |E| (V|E[ where E is theelectric field strength. This force, F, acts in the direction ofdecreasing electric field and accelerates both positive and negativecharges in the same direction. More particularly, the gradient fieldforces acting on electrons and ions are shown by:

where F is the force acting on the electrons, F is the force acting onthe ions,

e is the electron charge,

q is the ion charge,

m is the electron mass,

M is the ion mass,

or is the angular frequency of the signal.

By novel application of these gradient field forces, the bombardment ofthe window disc by electrons and ions can be substantially reduced to avalue much lower than is experienced in prior-art windows. Therefore, inaccordance with the concept of the present invention, the manner ofapplying such gradient field forces is determined by the design of thewindow dis-c support configuration.

Referring to FIGURE 2, there is shown an embodiment which applied thegradient field forces generated therein, as set forth by the presentinvention. More particularly, the bombardment-free microwave window 22is inserted where desired in a waveguide system and, in particularly,between lengths of rectangular waveguide 23 and 24. A dielectric windowdisc 26 is mounted in sealed, transverse relation within a substantiallength of circular waveguide 28 of proper dimensions; the waveguide 28thus acting as a support for the disc 26. Annular waveguide wall members30 and 32 are preferably brazed to, and extend generally radiallyoutward from, the circular support waveguide 28 to form an abruptwaveguide transition region at either end thereof. The wall members 30,32 are herein secured in perpendicular relation to window disc supportwaveguide 28; however, the angle therebetween could be acute or obtuse.Short lengths of circular waveguide 34 and 36 are coaxially aflixed tothe outer circumference of the radially extending wall members 30 and32, respectively. The lengths of waveguide 34 and 36 are connected tothe rectangular waveguides 23 and 24, respectively, by tapered waveguideportions 38 and 40, respectively. Thus, waveguide wall members 30, 32and integral waveguide lengths 34, 36, respectively, comprise means forcreating an abrupt transition region near the window disc 26. Thedimensions (diameters, widths, etc.) of the components forming thewaveguide window 22, as well as the window disc material and dimensions,are determined by the desired waveguide transmission characteristics;such matter being well known in the art and needing no furtherexplanation herein.

Therefore, in accordance with the present invention, there is provided arelatively abrupt window disc region; that is, a region which variesfrom a relatively small crosssect-ion at the window disc region, to arelatively larger cross-section. From such larger cross-section, thewindow then is tapered gradually to substantially the same smallcross-section as at the disc region. Therefore (FIGURE 2), the abrupttransistion region provides a large average field force F acting awayfrom the window disc 26, while the tapered transistion region provides arelatively smaller average field force F acting towards the window'disc.That is, since it is necessary to reduce the greater cross-sectionregion of waveguide portions 34 and 36 to the crosssectional dimensionsof the rectangular waveguide without creating time-average forces of amagnitude equal and opposite to those created by the abrupt transistion,gradually tapered sections (portions 38, 40) are provided between theabrupt transition regions and the rectangular waveguides.

However, it is possible to construct an embodiment wherein the angle ofthe taper of the tapered waveguide portions approaches zero. That is(generally in circular waveguide systems), the diameter of the systemwaveguides equals that of the lengths of waveguide 34 and 36. In suchcase, further described below, there is no force F acting in a directiontoward the window disc 26, and the optimum condition of having onlyaforce F acting awayfrom the disc is realized.

In general, particles which enter axially within rectangular waveguide23 or 24 from a point outside the window 22 are urged toward the windowdisc 26 by force F However, upon passing the neutral point between forceF and F (a point where the opposing fields cancel one another and arethus equal to zero), the particles are acted upon by the relativelylarger force F and are forced back away from the window disc region.

Thus, it follow that particles which originate at any point along theforce F as shown in the FIGURE 2, or, in essence, at any point outsidethe corners of the abrupt transistion region, will be forced away fromthe window disc 26 by force F However, any particles which originateinside the circular waveguide 28 will be unaffected by the force F, andwill pass through the transverse electric field lines within thewaveguide 28 to strike the window disc 26. Therefore, in the embodimentof FIGURE 2, although particles originating outside the waveguide 28 areprevented from bombarding the window disc 26, those particlesoriginating inside the waveguide 28 are not necessarily prevented fromso doing.

Therefore, referring to FIGURE 3, there is shown an alternativeembodiment of the present invention, wherein the length of the windowdisc support waveguide is reduced to the length of support waveguide 42equal to the width of the window disc 26. Thus, there is, in essence, noregion within support waveguide 42 wherein particles originated are notaffected by the force F Even particles originating at the corners of theabrupt transition region are forced in a direction away from window disc26', since there is no region near the window disc 26' surfaces whereinthe electric field lines exist parallel to the surfaces. That is,essentially all electric field lines bow outwardly away from the windowdisc surfaces, thereby creating an average field force F which acts awayfrom the disc.

FIGURE 4 depicts another embodiment of the present invention, wherein asecond abrupt transisition-tapered transition configuration 44 is addedto one side of the microwave window configuration of FIGURE 2. Theregion within the second abrupt transition-tapered transitionconfiguration 44 generates an additional force F which acts to furtherstrengthen the effects of the gradient field forces F acting away fromthe region of dielectric disc 26". It is to be understood that severalsuch additional abrupt transition-tapered transition configurations canbe added in series to either side of the disclosed embodiments; theaddition of such components being determined by the desired waveguidetransmission characteristics, -i.e., impedance matching, desiredfrequencies, etc.

FIGURE 5 shows a modification of the above embodiments, wherein theangle of convergence of the tapered portion thereof approaches zero.That is, there is no tapered waveguide portion. Thus, since there is, inessence, only an abrupt transition region in this construction, itfollows that there is created only an average field force F acting in adirection away from the window disc 26", and all particles are forced ina direction away from the disc 26.

While the invention has been disclosed with respect to severalembodiments thereof, it will be apparent to those skilled in the artthat numerous variations and modifications may be made within the spiritand scope of the invention. For example, the window construction, aswell as the waveguide system, could be of rectangular configuration, ofor circular configuration. Also, under particular internal pressureconditions, it is possible to utilize the present invention only on oneside of the window disc to thus protect from bombardment only one sidethereof. Further, it is to be understood that the abrupt transition ofprevious description does not have to be of design, but could define anangle greater or smaller than 90, e.g.,

70,etc. Thus, it is not intended to limit the invention except asdefined in the following claims.

What is claimed is:

1. A waveguide window for use within a rectangular waveguide systemwherein bombardment of a wave-permeable window disc by ions andelectrons is substantially reduced comprising a length of circularsupport waveguide, said wave-permeable window disc being supported insealed transverse relation within said length of circular supportwaveguide at the midpoint therein, an annular waveguide wall extendinggenerally radially outward from the ends of said circular supportwaveguide, a circular wave-guide wall member coaxially secured in sealedrelation to the outer circumference of said annular waveguide wall,wherein the outside diameter of said circular waveguide wall member isapproximately twice the diameter of the circular support waveguide, saidannular waveguide wall and circular waveguide wall member defining anabrupt transition region having an electromagnetic field configurationwith a time-average force acting in a direction away from said windowdisc, a length of tapered waveguide secured at its large end to saidcircular waveguide wall member and at its small end to the rectangularwaveguide of the waveguide system in sealed coaxial relation thereto,said tapered waveguides thereby inherently having an electromagneticfield configuration with a time-average force acting in a directiontowards the window disc but counterposed by a force created by theabrupt transition region which is greater than that of the taperedregion, resultingly causing particles approaching the waveguide windowto experience a net acceleration in a direction away from the windowdisc.

2. A waveguide system comprising adjacent waveguide sections underdifierential pressures, a wave-permeable window between the adjacentwaveguide sections to maintain the differential pressures, thecross-sectional area of said wave-permeable window being significantlymore than the cross-sectional area of the adjacent waveguide section, alength of support waveguide, said wave-permeable window being supportedin sealed transverse relation within said length of support waveguide, awaveguide wall extending transversely outward from an end of saidsupport waveguide, a waveguide wall member secured in sealed relation tothe outer periphery of said waveguide wall, said waveguide wall andwaveguide wall member defining an abrupt transition region having anelectromagnetic field configuration with a time-average force acting ina direction away from said window, a length of tapered waveguide securedat its large end to said waveguide wall member and at its small end toone of said adjacent waveguide sections of said waveguide system insealed relation thereto, said tapered waveguide thereby inherentlyhaving an electromagnetic field configuration with a time-average forceacting in a direction toward the window, but counterposed by a forcecreated by the abrupt transition region which is greater than that ofthe tapered region, resultingly causing particles approaching saidwave-permeable window to experience a net acceleration in a directionaway from the window.

3. A waveguide window according to claim 2, further including a secondwaveguide wall extending transversely outward from the small end of saidlength of tapered waveguide and in sealed relation thereto, a secondwaveguide wall member secured in sealed relation to the outercircumference of said second waveguide wall, said second wall and secondwall member defining a second abrupt transition region having anelectromagnetic field configuration with a time-average force acting ina direction away from said second wall, a second length of taperedwaveguide secured at its large end to said second waveguide wall member,and at its small end to one of said sections of waveguide of thewaveguide system in sealed relation thereto, said second taperedwaveguide resulting in an electromagnetic field having a time-averageforce acting in a direction toward said second wall, but counterposed bya force created by the second abrupt transition region of said secondwall and second wall member which is greater than that of the taperedregion of said second tapered waveguide, resultingly strengthening theacceleration of particles away from said wave-permeable window.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS7/1955 Great Britain. 10/1962 Great Britain.

ELI LIEBERMAN, Primary Examiner.

1. A WAVEGUIDE WINDOW FOR USE WITHIN A RECTANGULAR WAVEGUIDE SYSTEMWHEREIN BOMBARDMENT OF A WAVE-PERMEABLE WINDOW DISC BY IONS ANDELECTRONS IS SUBSTANTIALLY REDUCED COMPRISING A LENGTH OF CIRCULARSUPPORT WAVEGUIDE, SAID WAVE-PERMEABLE WINDOW DISC BEING SUPPORTED INSEALED TRANSVERSE RELATION WITHIN SAID LENGTH OF CIRCULAR SUPPORTWAVEGUIDE AT THE MIDPOINT THEREIN, AN ANNULAR WAVEGUIDE WALL EXTENDINGGENERALLY RADIALLY OUTWARD FROM THE ENDS OF SAID CIRCULAR SUPPORTWAVEGUIDE, A CIRCULAR WAVE-GUIDE WALL MEMBER COAXIALLY SECURED TO SEALEDRELATION TO THE OTHER CIRCUMFERENCE OF SAID ANNULAR WAVEGUIDE WALL,WHEREIN THE OUTSIDE DIAMETER OF SAID CIRCULAR WAVEGUIDE WALL MEMBER ISAPPROXIMATELY TWICE THE DIAMETER OF THE CIRCULAR SUPPORT WAVEGUIDE, SAIDANNULAR WAVEGUIDE WALL AND CIRCULAR WAVEGUIDE WALL MEMBER DEFINING ANABRUPT TRANSITION REGION HAVING AN ELECTROMAGNETIC FIELD CONFIGURATIONWITH A TIME-AVERAGE FORCE ACTING IN A DIRECTION AWAY FROM SAID WINDOWDISC, A LENGTH OF TAPERED WAVEGUIDE SECURED AT ITS LARGE END TO SAIDCIRCULAR WAVEGUIDE WALL MEMBER AND AT ITS SMALL END TO THE RECTANGULARWAVEGUIDE OF THE WAVEGUIDE SYSTEM IN SEALED COAXIAL RELATION THERETO,SAID TAPERED WAVEGUIDES THEREBY INHERENTLY HAVING AN ELECTROMAGNETICFIELD CONFIGURATION WITH A TIME-AVERAGE FORCE ACTING IN A DIRECTIONTOWARDS THE WINDOW DISC BUT COUNTERPOSED BY A FORCE CREATED BY THEABRUPT TRANSITION REGION WHICH IS GREATER THAN THAT OF THE TAPEREDREGION, RESULTINGLY CAUSING PARTICLES APPROACHING THE WAVEGUIDE WINDOWTO EXPERIENCE A NET ACCELERATION IN A DIRECTION AWAY FROM THE WINDOWDISC.